Fluidized bed jet mill

ABSTRACT

The method and apparatus disclosed relates to an improved jet mill in which the material to be ground is fed into a chamber or zone containing a jet that emanates from a nozzle with high velocity, the material being in the state of a compact bed and the outlet of the nozzle being located beneath the surface of the bed. The bed is preferably a &#39;&#39;&#39;&#39;moved compact bed&#39;&#39;&#39;&#39; arranged so that the material to be ground is fed sideways into the jet. A sifter or classifier for the coarse and fine material produced in the grinding operation is arranged above the bed of material in the grinding chamber or zone. The compact bed of material may be moved in a variety of ways, and the volume occupied by it is filled up to the greater part with compact granular material. This could be defined as the ratio of wholly occupied volume to the volume of the compact matter, or by the term &#39;&#39;&#39;&#39;compact current&#39;&#39;&#39;&#39; as commonly employed in the pneumatic transportation art.

Write States Patent 1 Kaiser [54] FLUIDIZED BED JET MXLL [21] Appl. No.:135,919

[30] Foreign Application Priority Data Aug. 14, 1970 Germany ..P 20 40519.0

[52] US. Cl ..24l/39, 241/5 [51] Int. Cl ..B02c 19/06 [58] Field ofSearch ..24l/l, 5, 18, 39, 241/40 [56] References Cited UNITED STATESPATENTS 3,311,307 3/1967 Lopker ..24l/39 X 1,948,609 2/1934 Andrews eta1 1 ..24l/5 1,791,100 2/1931 Lykken ..24l/5 2,072,492 3/1937 Anger.....24l/40 2,103,454 12/1937 Graemiger et a1 .241/40 45] May 22, 1973Primary ExaminerGranville Y. Custer, Jr. AttorneyDean S. Edmonds et al.

57 I ABSTRACT bed. The bed is preferably a moved compact bed arranged sothat the material to be ground is fed sideways into the jet. Asifter orclassifier for the coarse and fine material produced in the grindingoperation is arranged above the bed of material in the grinding chamberor zone The compact bed of material may be moved in a variety of ways,and the volume occupied by it is filled up to the greater part withcompact granular material. This could be defined as the ratio of whollyoccupied volume to the volume of the compact matter, or by the termcompact current" as commonly employed in the pneumatic trans portationart.

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IN VEN TOR FLUIDIZED BED JET MILL BACKGROUND OF THE INVENTION 1. Fieldof the Invention The present invention relates to jet mills and, inparticular, to the mill-type in which the material to be comminuted isfed into a gas jet of high velocity wherein such material isdisintegrated by interparticle impact.

2. Description of the Prior Art A jet mill and its operation accordingto the invention is distinct from known types of mills in which thematerial to be ground is speeded up in an injector or by means of a gasjet, and subsequently disintegrated by impact upon a baffle plate or bycollision with material from a jet of the same kind moving in counterdirection. Known types of jet mills have in their interior, in additionto the gas jet for obtaining size reduction, a circulating current ofthe disintegrating gas by which the material to be comminuted isconstantly refed to the gas jets, and the material which has beensufficiently reduced in size is separated and carried out of the mill.

The methods employed in these prior known jet mills, however, have thefollowing disadvantages:

They have an inherent, very high power consump-.

tion;

A very small quantity of material is brought into the jet, especially infine grinding processes that require high velocities and classificationof the ground material;

The jet energy is exploited very incompletely in actual grinding;

The efflux velocity of the jets at the nozzles is reduced by thecounterpressure of the classifier, and because of high turbulence in themill, the classifier or sifter operates inefficiently so that it isnecessary to run it an extremely high velocity requiring considerableconsumption of pressure to obtain sifting at a high fineness of grain.

Some of the drawbacks referred to are eliminated according to GermanPat. Specification No. 1,164,806, of which the present inventor is ajoint inventor. According to the disclosure of this German patent, thegas jets are directed from the top down onto the surface of a bed ofmaterial to be ground which is constantly renewed with the aid of amechanical transport means. The design provided in this patent, however,has presented some mechanical difficulties which up to the present havenot been overcome.

SUMMARY OF THE INVENTION In accordance with the present invention, thejet mill comprises a grinding chamber with a jet or jets mounted in itslower portion, the chamber providing a grinding zone in which thegranular material to be ground is comprised of a compact bed in whichthe gas jet nozzle is located and from which a gas jet emanates withhigh velocity. In this arrangement granular material is in the state ofa moved compact bed which is preferably moved sideways into the gas jetemanating from the nozzle which is located beneath the surface of thebed. The energy of the jet is used up to disintegrate the granularmaterial in the bed, and while a sitter or classifier may be arrangedabove the bed, it is not operated by the impetus or velocity of the gasjet.

The term moved compact bed of material as used herein includes allvarieties of motion of a granular material, the occupied volume of whichis filled up to the greater part with compact matter. It could bedefined as the ratio of the wholly occupied volume to the volume of thecompact matter or by the term compact current" as usually used inpneumatic transportation. Examples for the motion in the compact bedare: the displacing of a solid bed as a whole with the aid of a movedsupport such as, e.g., a conveying belt;

the displacing of the material within the solid bed by means of acontinuous conveyor, e.g., screw or chain conveyor, insofar as theformation of gaps in the material is prevented by a suitable design;

the slipping down of the contents of a bin by discharging;

the sliding down (in case of banking-up from below) or the shooting down(in case of unhindered movement) on an inclined chute or in an obliqueconveyor pipe;

the fluidized bed;

the pneumatic pipe transport in the compact current (with free path ofthe particles considerably shorter than the pipe diameter).

Tests carried out in connection with the invention have shown that therealization of a high-loading of the jet with material and the missingor lack of power consumption for producing the circulating currentincrease the grinding efficiency. On the other hand, the higherconsumption of mechanical or pneumatic energy by the separate sifter isinsignificant.

The scope of the basic invention comprehends a plurality ofconfigurations and arrangements such as, e.g., for wear protection,arrangement of nozzles, means for material circulation, sitters andpropellants which can be interconnected in a unit-composition manner,the most advantageous of which are described in connection with thedrawings.

According to an extended arrangement of the inventive jet mill, a wearprotection can be obtained by en-- larging the air volume above thesurface of the material bed so that air resistance and gravity aresufficient to retard the flying grains of material. The wear by impactupon the mill wall then will be tolerable. This arrangement isespecially suitable for very fine feed material because of the shortstopping distance of the grains in the air.

As a wear protection, an armour-plate may be provided, such as, e. g.,an easily exchangeable baffle plate arranged vertically to the directionof the jet and above the surface of the material bed. This isparticularly suitable as an additional provision in case of a preciousproduct that admits of only a small charge of material, or if the chargeoccasionallyis not present, e. g., if the mill is emptied.

BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the inventionare illustrated schematically by way of example in the accompanyingdrawings in which the figures are vertical, sectional views respectivelyand further defined as follows:

FIG. 1 illustrates an embodiment of a jet mill with vibrator and rotarylock with chambers, in which the mill nozzle is directed perpendicularlyupward;

FIG. 2 illustrates a .jet mill including a mill chamber having aperforated bottom for use in fluidizing the material being ground, themill including nozzles providing an intersecting jet system and astandpipe lock;

FIG. 3 illustrates a mill chamber provided with a horizontal jet nozzleand with a screw conveyor for effecting inner circulation of granularmaterial;

FIG. 4 illustrates ajet mill provided with a screw conveyor for feedingand inner circulation of material;

FIG. 5 illustrates a multi-jet mill provided with a material stirrer;

FIG. 6 illustrates ajet mill provided with a bucket elevator forcirculation of granular material and with a baffle plate located abovethe material bed in the grinding chamber;

FIG. 7 illustrates a jet mill provided with pneumatic means for feedingmaterial into the jet;

FIG. 8 illustrates a jet mill assembly including an air stream sifter (aspiral air classifier) in one unit;

FIG. 9 illustrates a stirrer-type sifter (with counteracting blades);

FIG. 10 illustrates a rising-tube sifter with several zigzag-shapedsifting channels arranged in parallel;

FIG. 11 illustrates a spiral air sifter with one rotating front wall andblades generating the rotary current;

FIG. 12 illustrates a sifter provided with airpropelling jet nozzles;

FIG. 13 illustrates a simple rising-tube sifter or classifier;

FIGS. 14 and 14a illustrate a combined jet mill rotortype classifierwith zig-zag-shaped sifting channels, material admission along part ofthe rotor circumference and with a screw conveyor for returning thecoarse material to the jet mill;

FIG. 15 illustrates a rotor with zig-zag-shaped separating channels andarranged for material admission along the entire circumference of therotor;

FIG. 16 illustrates a nozzle provided with a jet needle controlledpneumatically;

FIG. 17 illustrates a nozzle structure having; an adjustable slot;

FIG. 18 illustrates a nozzle structure having an annular slot, and

FIG. 19 illustrates a jet mill provided with a baffle plate adapted toact as a level tracer.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the jet mill, represented inFIG. I, a single nozzle 3 having a perpendicular axis is located at thefoot of the funnel-shaped lower part 1 of the mill bin or grindingchamber 2, which is supplied with compressed air from an air chamber 4.The feed material to be ground is brought through a charging hopper 6, arotary lock with chambers 7 and a flexible delivery tube 8 into thespring-mounted bin 2 which is oscillated by a vibrator 9, thus causingthe material to slide continuously downward into the cavity swept freein the bed 5 by the strong jet. This embodiment, in which the jetemanating from the nozzle is vertical and retarded by the material bed,is particularly advantageous since it allows rather small bin dimensionsand mill contents without the risk of locating the inner wall of thebin, the feeding-t In the embodiment shown in FIG. 2, a nozzle plate 10of the jet mill carries nozzles 3 placed in such a way that theirjets'collide with each other at an acute angle. The nozzles are suppliedwith compressed air from the air chamber 4. The material to be ground,suitable to be fluidized, is brought into the mill chamber 2 throughcharging hopper 6 and a standpipe 11, and is fluidized byair which flowsout of the annular air chamber 12 through the porous bottom 13 extendingaround the nozzle plate 10 into the material bed 5, thus causing thematerial of the bed to continuously flow into the jets. Two or morenozzles may be mounted in the plate 10, but at an acute angle to acontrol axis so that one nozzle is not damaged by grains from thejet ofanother nozzle.

In FIG. 3 a jet mill with a horizontal nozzle is shown in which thematerial bed 5 is dimensioned so high and wide that no particle ofmaterial can be forced through the material bed by the kinetic energy ofthe jet. In this case when a sheet-metal plate was fixed above thematerial bed, it did not show any wear phenomena. The mill contents iscirculated by a screw conveyor 14 which presses the material sidewaysinto the gas jet coming horizontally out of the nozzle 3, that issupplied with compressed air from air chamber 4.

In FIG. 4 the material is passed into the mill bin by a screw conveyor14 which subsequently forces it sideways into the jet from nozzle 3,blowing perpendicularly upward, and being supplied with compressed airfrom air chamber 4. Material continuously slides from the bed towardsand into the screw conveyor 14 which continuously forces materialinwards into the jet as indicated by the sloping surface 16 of thematerial bed. A bin wall 15 is made conical in its lower part, as showndiagrammatically, in order to reduce mill contents. It is, of course,possible to furnish this jet mill and as well those which are shown inFIGS. 1 and 3 with several nozzles and, respectively, with pairs ofnozzles, and eventually with several screw conveyors provided that thelower part of the mill chamber is of adequate size.

FIG. 5 illustrates an embodiment suitable as a largescale jet mill. Itis provided with a stirrer l7 having a vertical axis 18 or drive shaft.The nozzles 3 of this mill which are supplied with compressed air froman air chamber 19, are placed as a ring around the stirrer 17. Thematerial for the bed 5 is fed to the mill by the controlled double swingflap 20, of any known kind, through the duct 21.

The material 5 to be ground by the jet mill of FIG. 6 is carried upagain and again by the bucket elevator 22 to the chute 23 on which itslides down hitting sideways the upward blowing jet of the nozzle 3which is supplied with compressed air from the air chamber 4. This millis furnished with an easily exchangeable wearprotecting or absorbingbaffle plate 24 mounted vertically above the direction of the jet andabove the bed of material in the grinding chamber. Raw material may besupplied to the bucket elevator.

In FIG. 7 ajet mill is shown in which the feed material is conveyed by apneumatic transport means as a compact current (piston conveyance) andis injected sideways into the upwardly blowing jet issuing out of thenozzle 3 that is supplied with compressed air from the air chamber 4.The material to be ground is fed through a charging hopper 6 and therotary lock with chambers 7 into a storage bin 25 from where it isbrought out by a blow-through rotary cell lock 26 into a duct 27 whichis supplied with compressed air through air inlet mouth 28. From theduct 27 the material is injected sideways into the jet from nozzle 3.The transport means effects both feeding and inner circulation. Theheight of the material surface 16 inside the mill bin 2 is kept at aconstant value by an overflow duct 29 which connects mill bin 2 withstorage bin 25.

A separator, sifter or classifier, such as illustrated in FIGS. 9 to 15,may be mounted on the mills of FIGS. 1 to 7. Such a combination unit isillustrated in FIG. 8 in which a stationary air separator is mounted atthe top of the jet mill. As it is known, such an air separator or sifterconsists, in the main, of a flat cylindrical sifting chamber 70 and theadjustable guide vanes 30. In the same way as shown in FIG. 4, thematerial to be processed is passed into the mill by a screw conveyor 14and subsequently forced sideways into the jet from nozzle 3, blowingperpendicularly upward, and being supplied with compressed air from airchamber 4. The coarse material being separated within the spiral airsifter falls back through duct 31 to the screw conveyor which forces itinto the jet again, whereas the fine grained material is carried away atthe top of the sifter in the air stream through duct 32 for finalseparation. For the same reasons as given for FIG. 4, the bin wall 15opposite the screw conveyor is made conical.

FIG. 9 illustrates a stirrer type sifter, of particular advantage forground material of medium fineness, which alternatively can be combinedwith any of the jet mills according FIGS. 1 to 7. The rotatable stirrer33 is, in this case, fitted with an exchangeable baffle plate 24 forwear protection. The coarse material thrown out from the stirrer 33moves along the inside of wall 34 of the sifting chamber back into thebin of the jet mill beneath the sifter, whereas the fine materialseparated by the sifter is carried away through a duct 35 for laterrecovery from the air stream. The stirrer is known from scatter typewind sifters as having counter-acting blades. Its task is to put the airascending in a cylindrical separating chamber into rotation.

FIG. illustrates a rising tube sifter or classifier, with severalzig-zag-shaped sifting channels 36 arranged in parallel, which is mainlyuseful in cooperation with a mill for coarse grinding. The coarsematerial in the sifter tubes simply falls back into the mill bin below.The fine material discharging at the top of the sifting channels iscarried away for recovery through duct 37. A sifter of this type is,e.g., known from German Pat. No. 1,482,427, or British Pat. No.1,014,723.

The sifter of FIG. 11 is a spiral air sifter with one rotating frontwall 38 and blades 39 for initiating the air rotation as in German Pat.No. 1,507,466. The coarse material in this sifter is simply thrown backinto the bin of the jet mill beneath the sifter. The fine materialdischarged centrally from the top of the sifter is carried away througha duct 40.

FIG. 12 illustrates a stationary spiral air sifter in which rotation iseffected by propelling jets 41. This sifter which makes possibleclassification to very high fineness, flings the separated coarsematerial back to the bin of the jet mill without the aid of movablestructural parts. The separated fine material is centrally discharged atthe top of the sifter and carried away through duct 42.

FIG. 13 shows a simple rising tube sifter 43 with a single siftingchannel or chamber designed for mounting on ajet mill. The coarsematerial separated in this sifter simply falls back into the bin ofjetmill. The fine material is discharged through the conically contractedupper part 44 of the sifter and the duct 45.

FIGS. 14 and 14a show in two views a sifting rotor 47, like that of theapplicants U. S. Pat. No. 3,089,595, with radial zigzag-shaped siftingchannels 48, rotating round a horizontal axis, combined with a jet millaccording to the present invention in which the material to be ground ispassed into the mill by the screw conveyor 14, as shown for example inFIG. 8, and subsequently is forced sideways into the jet of nozzle 3which has an upward direction and which is supplied with compressed airfrom air chamber 4. The classifier rotor 47, by which particleseparation of the highest fineness and selectivity can be achieved, isfed from the mill along section 49 to the circumference of the rotor.The coarse material thrown out along section 50 of the rotorcircumference falls back through the duct 51 to the screw conveyor 14which forces it again into the jet 3, whereas the time materialseparated by the rotor is carried away centrally through duct 52.

The feed of ground material to the sifting rotor, rotating on the axis53, with radial zig-zag-shaped sifting channels 48, shown in FIG. 15, ismaintained along the whole circumference 54 of the rotor. The separatedcoarse material is thrown back into the chamber 55 from which it isreturned to the jet mill. The fine material is carried away centrally ofthe rotor and discharged through a duct 56. An additional narrow bladewheel 57 is placed between the upper face of the rotor and the casing,the blades of which, having recesses 58, are closely matched with thecollars 59 on casing wall 60. This blade wheel 57 is provided to preventmaterial from passing through the gap between the upper face of thesifting rotor and the casing wall 60.

FIG. 16 illustrates a closing means for the nozzles in the form of a jetneedle 62 preferably controlled by a pneumatic switch or actuating means61. This closing means prevents material from falling into the nozzleswhen the mill is stopped or when the supply of grinding gas fails.

The slot nozzle 63, shown in FIG. 17, which can be manufactured morecheaply and exactly than many small nozzles, allows the width of theslot 64 to be adjusted by shifting a plate 65 with the aid of a screw66. The compressed air is supplied to and distributed by air chamber 4.The length of the slot is selected according to the jet area of themill.

FIG. 18 illustrates a form of annular slot nozzle which can be used inmills where the material to be ground is fed to the nozzle symmetricallyfrom outside, e.g., by fluidization, or from inside, e.g., by a stirringapparatus.

The nozzle structure is readily understood from the showing and may beused in place of a number of small nozzles.

FIG. 19 shows a mill provided with a baffle plate 67 acting as leveltracer which is mounted above the material surface 68. A signal orcontrol switch 71 is oper ated by the tracer arm if the impellent powerof the ascending jet of air and material acting on the baffle plateexceeds a definite value, that can be fixed by adjusting the position ofa sliding weight 69. The level tracer shown in FIG. 19, or other typesof level tracers, such as radioactive emitters or weighing types, may beused so that it is possible to control the level of the body of materialin the mill chamber and correlate it with the said return and grindingoutput. The control 71 may be used to actuate the feeder which may alsobe responsive to the return grinding output. In a non-fluidized bed ofmaterial in the grinding chamber, the jet discharging from the nozzle isretarded in the bed of material. If the baffle plate 67 is affected by aforce, it is an indication that the jet may be breaking through thematerial bed because the level is too low. This force will actuate theswitch 71 and cause an increase in the feed return.

In the preferred type of mill according to the present invention, afluidized bed is employed and the material ascends therefrom upwardly ina strong jet. Tests have shown that a baffle plate such as that shown inFIG. 19, located at a predetermined height, receives an impellent powerfrom the jet which is the greater, the higher the level of material inthe chamber. If there is a decrease of power applied to the baffle platebelow a certain point, this point may serve as a signal point foroperating the control 71.

The material in the mill must generally be fed to the jet several timesto complete reduction. The mill, therefore, runs with a high innercirculation rate by which the unground material thrown out by the jet isconstantly re-fed to the jet. Also the coarse material that had beenrejected by the sifter is brought back to the jet. Consequently, thefollowing currents of material are circulating in a mill unit accordingto the present invention:

1. The material fed to the mill.

2. The transport of material of the bed into the jet, the conveyanceinside the jet and the discharge back to the material bed.

3. The discharge out from the material bed to the classifier (caused bythe above-described fountains of material without any additionalprovision).

I 4. The reflux of the coarse material back to the material bed or intothe jet.

5. The flow of the fine material from the classifier to a separator.

The currents numbered 2, 3 and 4 represent the inner circulation.

To obtain this circulation, any suitable conveying devices can be used.For example, the material may be fed into the jet by a stirrer thatnecessarily must be designed in such a manner that it never can be hitby the jet. Expecially in case of large-scale mills a stirrer with aperpendicular axis and nozzles spaced around said stirrer may be used asillustrated in FIG. 5. Such a ring of nozzles that has a diameter of lm, in which the nozzle diameter is 5 mm and the circular pitch is mm,yields a clear opening of the nozzles of 63 cm by which, at a throughputof compressed air of 38 tons per hour and a jet power of about 1,300 kWcan be obtained.

The advance in the art provided by the jet mill of the present inventioncan be recognized from typical test results presented as follows:

In this test a jet mill according to FIG. 4 with a bin diameter of 630mm was operated in combination with a sifter according to FIG. 15, thathas an outside diameter of 400 mm, a sifting rotor with zigzag-shapedradial sifting channels, revolving at a speed of 2,000 rpm. Limestonepower was used as test material, having been obtained as the finematerial from a usual wind sifter, with 50 per cent minus 12 microns anda specific surface of 4,000 Blaine. For comparison, part of the samematerial was ground in a conventional spiral jet mill of the type knownfirst under the trademark MI- CRONIZER," having the characteristicfeatures: Outside diameter of the grinding chamber 200 mm, six nozzleseach with an opening of 2.5 mm, one injector nozzle with an inner widthof 3.8 mm.

To determine the expenditure for the grinding process, the jet outputthat could be obtained by adiabatic expansion, was calculated out fromthe measured pressure of the compressed air and the cross section areaof the nozzles. It is represented by the intelligible unit jet-kW."

To determine the grinding result the specific surface of the materialbefore and after milling was evaluated by Blaine Test and therefrom thenew surface was computed that was produced during 1 hour. intelligibleunit is km lh.

The grinding efficiency is defined by the specific power consumption injet-kwhlkm which is the ratio that the expenditure for the grindingprocess bears to the grinding result.

MILL TYPE Conventional According to the Invention Day/No. of test 3 l.7701] 4.8.7011 Pressure of compressed air 6 5.5 (kg/cm over airpressure) Throughout of air g/h) 24 730 Jet output (jet-kW) 8.3 28.8Yield of material (kg/h) 15 63 Specific surface of 8000 9700 material(cm/g) Increase of surface (cm lgl 4000 5700 Specific power consumptionI370 690 (jet-kWh/km) The height of material bed above the nozzle outletin the new jet mill during the test was about 500 mm, the bed beingfully fluidized with the material moving by itself to the nozzle. Themixture of air and material ascended as a strong jet with a diameter ofabout 5 cm vertically upward out from the bed, but at such a lowvelocity that no wear of the paint on the sifting rotor was visible atthe area of impact. Wear was perceptible only on the periphery of thesifting rotor because of its circumferential speed. The values of masstransportation and velocity of the ascending jet, however, could beestimated inexactly at 35 t/h and 2.5 5 m/sec.

The gaseous grinding or fluid agent for disintegrating granular materialby inter'particle impact according to the present invention may becompressed where as described either at ambient temperature or in aheated or cold condition. Water vapor at any suitable temperature orinert gas may be used. Since the jet mill according to this inventionmay be built up in a size that allows working with several thousandkilowats, the outlet of a gas turbine may be employed as the gaseousmedium in a compressed and heated condition. A gas turbine would havemany advantages over the use of steam boilers or internal combustionengines while at the same time providing an effective grinding aid.

The jet mill according to the present invention allows the material bedto be rated in height and width so large that the kinetic energy of thejet is unable to force any grain through the whole bed. For this purposethe mill may be filled up with so much material that wear, even on aplate above the bed, is not noticeable. A surplus material in the mill,that has no detrimental effect at all, may represent a valuable reservein case of varying conditions of work. With a high bed as described thedisintegrating gas emerges bubble-shaped throwing up fountains ofmaterial as in a fluidized bed. As these fountains are not produced bythe kinetic energy of the jet, but by the pressure drop within thematerial bed, they move so slowly that they virtually cause no wear inthe mill. This method of working presents the advantage of eliminatingany wear phenomena in the jet mill and of fully utilizing the kineticenergy of the jet and of the accelerated particles for interparticleimpact and for size reduction. In addition, there is a minimum of noiseeffects. In most cases it is not necessary to pile up excess material inthe mill in order to use up the whole kinetic energy of the jet and ofthe particles of the material acted on by the jet.

In a non-fluidized bed the jet creates a cavity that is entered from theside by the grains of the material being conveyed by the transportingsystems that are provided for the process of circulation in the millchamber. These grains are accelerated by the jet and, at a sufficientheight of the bed, they are thrust entirely into the unmoved part of thebed, thereby effecting the process of size reduction by consumption ofthe kinetic energy. The impellent power of the jet presses against thelower part of the bed, thereby diminishing the pressure of it upon thebase of the mill. The weight of the bed within the range of action abovethe jet (e.g., a cone with an aperture with twice the size of the angleof friction) should be at least equal to the impetus of the jet, thusdetermining the minimum height of the bed. In practice it mustnecessarily be higher according to a certain safety factor to preventshooting through gaps and chimneys.

Depending upon the kind of material being processed, the gas jet eitherascends after being retarded, in diffuse state between the grains orforms clefts and chimneys from which grains may secondarily fly upwards.Their speed is, however, so low that they do not cause any wear sincethe only accelerating power acting upon them is the fall of pressureinside the bed.

The phenomena in a fluidized bed of material are quite different.Immediately after discharging from the nozzle, the jet sucks up thesurrounding, fluidized material and immediately after passing a shortdistance the jet is saturated with the material; and, under theseconditions, the main part of size reduction by inter-particle impact islikely to have taken place. Compared to the small cross-sectional areaof the jet, the jet velocity is yet rather high. As more material issucked in, the speed of the jet decreases further on and thecrosssectional area increases. Contrary to the efflux of an air jetunder water, where the airjet is broken up into bubbles under theinfluence of turbulence and surface tension, the jet under considerationretains its compactness along a considerable distance and it comes outfrom the fluidized material bed in a compact state. Though this emergingjet may still carry a large amount of the impellent power of theoriginal gas jet, its velocity is so low and its mass so voluminous thatit is no longer capable of causing any wear. Therefore, in a fluidizedbed it is enough to pile up the material so high that the jet ofmaterial still ascending with part of the impetus of the original gasjet does not cause any wear at any area of contact with the millsurface. This relation to the spot of striking is important for thereason that the jet emerging from the bed already with a low speed isfurther retarded by gravity.

I claim:

1. A jet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the nozzle means to provide the jet, and means for moving granularmaterial to be acted on by the jet into a side of the jet of gaseousmaterial issuing from the nozzle means.

2. A jet mill as claimed in claim 1, wherein the nozzle means is set todirect a jet of gaseous material vertically into and through the bed ofmaterial in the chamber.

3. A jet mill as claimed in claim 1, wherein the nozzle means isprovided with means for automatically closing its outlet in response tothe stopping of the delivery of gaseous material thereto.

4. A jet mill as claimed in claim 1, wherein the nozzle means includesmeans for delivering a plurality of converging jet streams which collideat an acute angle in the bed of material.

5. A jet mill as claimed in claim 1, wherein the nozzle means includes aplurality of upwardly directed nozzles arranged in a ring, and a stirrerlocated inside the ring of nozzles for moving granular material into thejets issuing from the ring of nozzles.

6. A jet mill as claimed in claim 1, including means for introducing anddistributing a gaseous means into the lower portion of the bed ofmaterial in the grinding chamber to fluidize the bed.

7. A jet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the nozzle means to provide the jet, a feed bin, an overflow from themill chamber into the feed bin, and means for supplying granularmaterial from the bin into the side of the jet of gaseous materialissuing from the nozzle means.

8. A jet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the nozzle means to provide the jet, means for moving granularmaterial to be acted on by the jet into a side of the jet of gaseousmaterial issuing from the nozzle means, and means for vibrating the bedof granular material in the grinding chamber.

9. A jet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the 10. A jet mill as claimed in claim 9, wherein the level tracerincludes a baffle plate located in the upper part of the grindingchamber and acted on by the jet issuing upwardly from the body ofgranular material in the granular material bed in the mill chamber, anda conchamber.

trol actuated by the movement of the level tracer.

1. A jet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the nozzle means to provide the jet, and means for moving granularmaterial to be acted on by the jet into a side of the jet of gaseousmaterial issuing from the nozzle means.
 2. A jet mill as claimed inclaim 1, wherein the nozzle means is set to direct a jet of gaseousmaterial vertically into and through the bed of material in the chamber.3. A jet mill as claimed in claim 1, wherein the nozzle means isprovided with means for automatically closing its outlet in response tothe stopping of the delivery of gaseous material thereto.
 4. A jet millas claimed in claim 1, wherein the nozzle means includes means fordelivering a plurality of converging jet streams which collide at anacute angle in the bed of material.
 5. A jet mill as claimed in claim 1,wherein the nozzle means includes a plurality of upwardly directednozzles arranged in a ring, and a stirrer located inside the ring ofnozzles for moving granular material into the jets issuing from the ringof nozzles.
 6. A jet mill as claimed in claim 1, including means forintroducing and distributing a gaseous means into the lower portion ofthe bed of material in the grinding chamber to fluidize the bed.
 7. Ajet mill for comminuting or grinding granular materials byinter-particle impact, comprising a grinding chamber containing a bed ofgranular material to be disintegrated, means for supplying granularmaterial to the grinding chamber to maintain the bed, an upwardlydirected nozzle means located in the lower portion of the chamber fordelivering a high velocity jet of gaseous material into the bed ofgranular material in the chamber, means for delivering gaseous materialto the nozzle means to provide the jet, a feed bin, an overflow from themill chamber into the feed bin, and means for supplying granularmaterial from the bin into the side of the jet of gaseous materialissuing from the nozzle means.
 8. A jet mill for comminuting or grindinggranular materials by inter-particle impact, comprising a grindingchamber containing a bed of granular material to be disintegrated, meansfor supplying granular material to the grinding chamber to maintain thebed, an upwardly directed nozzle means located in the lower portion ofthe chamber for delivering a high velocity jet of gaseous material intothe bed of granular material in the chamber, means for deliveringgaseous material to the nozzle means to provide the jet, means formoving granular material to be acted on by the jet into a side of thejet of gaseous material issuing from the nozzle means, and means forvibrating the bed of granular material in the grinding chamber.
 9. A jetmill for comminuting or grinding granular materials by inter-particleimpact, comprising a grinding chamber containing a bed of granularmaterial to be disintegrated, means for supplying granular material tothe grinding chamber to maintain the bed, an upwardly directed nozzlemeans located in the lower portion of the chamber for delivering a highvelocity jet of gaseous material into the bed of granular material inthe chamber, means for delivering gaseous material to the nozzle meansto provide the jet, means for moving granulaR material to be acted on bythe jet into a side of the jet of gaseous material issuing from thenozzle means, a level tracer for controlling the level of the granularmaterial bed in the mill chamber, and a control actuated by the movementof the level tracer.
 10. A jet mill as claimed in claim 9, wherein thelevel tracer includes a baffle plate located in the upper part of thegrinding chamber and acted on by the jet issuing upwardly from the bodyof granular material in the chamber.